Radar can operate in either monostatic or bistatic form. In either case, a radar pulse is emitted by a transmitting antenna, scatters off a target, and the scattered pulse is received by a receiving antenna. In monostatic radars, the transmitting and receiving antennas are the same, and the only reflected signal received is the signal that reflects back in the original direction. In bistatic radars, the transmitting antenna and the receiving antenna are separated by a distance that is comparable to the expected target distance.
In monostatic radars, the portion of the scattered signal that is received by the original antenna is known as radar backscatter, and is characterized by a backscatter radar cross-section (RCS).
The backscatter RCS of a radar signal has many uses. For example, the position of the target can be determined from the round-trip propagation time. Other characteristics such as velocity and size of the target may also be determined by analysis of the scattered radar signal.
Solid conducting spheres have been used as radar calibration targets because they have a backscatter RCS that is well known in terms of analytic solutions and because this RCS is independent of viewing direction. See P. A. Bernhardt, T. Ainsworth, K. Groves, T. Beach, R. G. Caton, C. S. Carrano, C. M. Alcala, and D. D. Sponseller, “Detection of ionospheric structures with L-band synthetic aperture radars,” Proceedings of the 2008 IEEE Geoscience and Remote Sensing Symposium, Boston, Mass., Jul. 6-11, 2008; H. Liu and J. Zou, “On unique determination of partially coated polyhedral scatterers with far field measurements,” Inverse Problems 23, 297-308, 2007; P. A. Bernhardt, C. L. Siefring, J. F. Thomason, S. P. Rodriquez, A. C. Nicholas, S. M. Koss, C. Hoberman, and D. L. Hysell, “The Design and Applications of a Versatile HF Radar Calibration Target in Low Earth Orbit,” Radio Sci., 43, RS1010, doi:10.1029/2007RS003692, 7 Feb. 2008; and G. T. Ruck, D. E. Barrick, W. D., Stuart, C. K. Krichbaum, Radar Cross Section Handbook, Plenum Press, New York, 1970.
Wire grids have been used to approximate solid objects. See H. Liu, et al., supra and Bernhardt and Siefring, et al., supra. See also A. C. Ludwig, “Wire Grid Modeling Of Surfaces,” IEEE Transactions on Antennas and Propagation, 35, 1045-1048, 1987; C. W. Trueman and S. J. Kubina, “Fields Of Complex-Surfaces Using Wire Grid Modeling,” IEEE Transactions On Magnetics, 27, 4262-4267, 1991; and A. Rubinstein, F. Rachidi, and M. Rubinstein, “On wire-grid representation of solid metallic surfaces,” IEEE Transactions On Electromagnetic Compatibility, 47, 192-195, 2005.
Several techniques have been attempted to make an object a less visible scatterer of electromagnetic radiation such as radar waves.
In a first technique, the object can be covered in materials that absorb the incident radar signals. For example, radar cross section reduction may be achieved over a wide band by placing a lossy dielectrics and radar absorbing materials (RAM) around perfectly conducing spheres. See, e.g., B. Kolundzija, J. Ognjanovic, M. Tasic, D. Olcan, D. Sumic, M. Bozic, M. Kostic, and M. Pavlovic, WIPL-D Software Users Manual, WIPL-D d.o.o., Belgrade, 2010; B. Chambers and A. Tennant, “Optimized design of Jaumann radar absorbing materials using a genetic algorithm,” IEE Proceedings—Radar Sonar And Navigation, 143, 23-30, 1996; and H. C. Strifors and G. C. Gaunaurd, “Scattering of electromagnetic pulses by simple-shaped targets with radar cross section modified by a dielectric coating,” IEEE Transactions On Antennas And Propagation, 46, 1252-1262, 1998.
In another technique, the object may be shaped or contoured so that most radar of the radar signal is deflected away from the radar source. See M. Skolkik, “Echo Reduction,” Radar Handbook, 2nd Ed., Section 11.5, pp. 11.43-11.51 (1990).
Third, a radar target can be screened by materials that scatter the radar signal before hitting the target and create false targets to confuse the interpretation of the radar data.
To this list of absorption, deflection, and screening is added the fourth so-called “cloaking” technique where the radar signal passes around the target region without either deflection or absorption. H. Mosallaei and Y. Rahmat-Samii, “RCS reduction of canonical targets using genetic algorithm synthesized RAM,” IEEE Transactions on Antennas and Propagation, 48, 1594-1606, 2000. Deflection produces reduced backscatter with primarily oblique scattering to the sides. Absorption provides reduced radar cross section (RCS) in all directions. True cloaking yields mainly forward scatter similar to scatter from empty space so that no shadows are formed in the forward direction.
The combination of spherical wire grids around solid spheres has not been previously considered for applications to reduce target identification by radar scatter.